The present invention relates to a device and a method for transferring wafers to and from FOUP type cassettes.
A device of this type is disclosed in WO 96/26874.
For transporting wafers use is made of cassettes provided with a cover fitted such that it is removable. Said cover can be provided to a greater or lesser extent with a gastight seal with respect to the other part of the cassette. If the wafers have, for example, to be treated in a furnace, the wafers are removed from the cassette and placed in a wafer rack and said wafer rack is then introduced, together with the wafers, into the furnace. After the treatment the wafers together with the rack are removed from the furnace and the wafers are transferred back to the cassette and transported for further treatment.
In general, treatment devices such as furnaces are under a controlled atmosphere, for example an inert nitrogen atmosphere, during loading or removal. It is necessary also to bring the wafers: into such an atmosphere. Numerous proposals are disclosed in the prior art for bringing the wafers into the controlled atmosphere of the treatment device before said wafers enter the chamber in which the treatment device has been arranged.
A first proposal comprises fitting a connection on the cassettes. Said connection engages, in the cassette storage device, which storage device can contain several tens of cassettes, on a coupling piece arranged in the storage device and, after connecting the cassette and the storage device, purging with the gas concerned takes place through said connection. Because the wafers fit accurately in the cassettes, it has been found that a completely homogeneous atmosphere can be achieved only with very great difficulty. A very large quantity of gas is required for this, whilst under normal conditions it cannot be guaranteed that every part of the cassette chamber has been provided with the controlled atmosphere.
Another proposal is not to remove the wafers directly from the cassette in order to place these in a wafer rack but to provide a lock construction in which the cassette is accommodated in its entirety. In a lock chamber of this type the atmosphere is then changed to the controlled atmosphere. It has been found that the various operations are time consuming, whilst the chamber for accommodating the cassette in turn adds additional floor area to the complete treatment device.
A construction having a lock chamber is described in WO 96/26874.
This construction is provided with a single closure which closes off both the first and the second opening. With this arrangement the first opening is made in a component that is movable with respect to the rest of the construction. Opening is effected by moving said movable component away from the closure.
This construction has the disadvantage that the lock chamber has a moving part. As a result of the movement, dust or other particles can be released which give rise to undesirable contamination. Furthermore, this construction is bulky and complex. Consequently the space requirements for such a lock construction is appreciable. This plays a role especially when larger wafers are used. Moreover, as a consequence of the possibility of part of the wall of the lock chamber being movable, the lock chamber has a relatively large volume, which makes purging thereof more difficult. Furthermore, it is a disadvantage that movement of the cassette relative to the lock chamber takes place during the operation for loading/unloading the cassette.
The aim of the present invention is to avoid these disadvantages and to provide a simple construction with which communication with the lock chamber can be achieved without moving the cassette and with which it is possible to purge the cassette chamber separately from the surroundings in a simple manner.
By using two separate closures in the lock chamber it is possible to close off both the first and the second opening individually. Moreover, it is no longer necessary to fit moving wall sections in the lock chamber. The construction according to the invention can be made appreciably more compact, as a result of which the volume of the lock chamber is reduced. Consequently the space requirement for the entire treatment device can be reduced. Furthermore, it is then possible in a simple manner to pass gas through both the lock chamber and the cassette using a relatively small quantity of gas. Said gas can be a purging gas. Preferably, the supply means for such a gas are constructed such that the stream of gas initially moves into the space in the cassette.
According to the invention the cassette is preferably a cassette of the so-called FOUP type. In this case the cassette rack is mounted such that it is fixed in the housing and the wafers are removed from the cassette in the direction of the plane thereof. This is in contrast to a so-called SMIF cassette, with which the wafer rack is mounted such that it is movable in the housing and the wafers, together with the wafer rack, are removed from the cassette in the direction perpendicular to the plane of the wafers.
With the aid of the invention it is possible to carry out purging via a cover for the cassettes. By initially only partially opening the cassette cover it is possible, on the one hand, to obtain good access to all levels of the wafers and, on the other hand, the quantity of purging fluid is restricted as far as possible. If use is made of a lock construction close to the opening it is possible to subsequently remove the cover completely.
Preferably, the closures described above are constructed as flat plates, which both move away to the side.
The invention also relates to an assembly consisting of the device described above and a cassette, the cassette being constructed as a FOUP cassette.
The invention also relates to a method for transferring wafers from/to cassettes in a lock chamber, wherein the cassette is positioned with the side to be opened against the lock chamber, such that the closure of the cassette is coincident with a closure for a first opening of said lock chamber, after which the closure of the cassette, together with the closure of the lock chamber, is removed into the lock chamber, which lock chamber is provided with a further closable opening for removing/feeding said wafers from/to the lock chamber, wherein during the removal of the closure of the cassette the further opening is closed and a purging gas is fed through said lock chamber, followed by opening of said further opening.
The invention will be explained in more detail below with reference to an illustrative embodiment shown in the drawings.